During the past several years we have been studying the inductive tissue interactions which lead to embryonic lens formation. The role of the optic cup in induction of the lens has been intensively studied and lens induction is often cited as an example of how cell determination occurs in vertebrates. Other inductive interactions, however, which precede contact of the presumptive lens ectoderm with the optic vesicle, have also been implicated in lens formation. Our work has shown that the optic vesicle alone is a very weak inductor of the lens, and that the early inductors are required to establish a lens-forming bias in head ectoderm as part of lens determination. While the evidence is compelling that early inductors must exist, we do not believe that the source of these signals has been clearly established. Or recent data supports a model in which the early steps of lens determination are intimately linked with induction of the nervous system, particularly with the formation of the eye primordium during gastrulation. Using a simple explant system we have recently devised to study the induction of both eye and lens, we propose experiments to test this model, setting as our major goal in the next several years to clarify the interactions that specify eye and lens formation. In addition we propose to initiate experiments for identifying the intercellular signalling mechanisms leading to lens and eye determination and for understanding how the ability of ectoderm to respond to these signals is regulated.